[UNHELPFUL ERROR MSG] obscure panic on closed connection

[ngortheone]

I have accidentally closed the db connection before using it ( I had a stray defer db.Close() )

It took me a while to figure out what was causing the panic - there was no good error message to tell me what was the problem.
The panic message was looking like this:

 --- FAIL: TestCreateAndInitializeDatabase (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0x574bfa]

goroutine 7 [running]:
testing.tRunner.func1.2({0x7533c0, 0xa5cbd0})
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/testing/testing.go:1545 +0x238
testing.tRunner.func1()
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/testing/testing.go:1548 +0x397
panic({0x7533c0?, 0xa5cbd0?})
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/runtime/panic.go:914 +0x21f
modernc.org/libc.(*TLS).Alloc(0xc000181040?, 0x7f5f21600428?)
	/home/runner/go/pkg/mod/modernc.org/libc@v1.29.0/etc.go:248 +0x1a
modernc.org/sqlite/lib.logBadConnection(0x0, 0x79c9ed)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:14577 +0x32
modernc.org/sqlite/lib.Xsqlite3SafetyCheckSickOrOk(0x5dfdc5?, 0xc000102820?)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:14619 +0x38
modernc.org/sqlite/lib.sqlite3Close(0xc00018d870?, 0x7f5f21600428, 0x0)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:115266 +0x2e
modernc.org/sqlite/lib.Xsqlite3_close(...)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:115332
zombiezen.com/go/sqlite.(*Conn).Close(0xc000074370)
	/home/runner/go/pkg/mod/zombiezen.com/go/sqlite@v1.1.0/sqlite.go:212 +0x11d
panic({0x7533c0?, 0xa5cbd0?})
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/runtime/panic.go:914 +0x21f
modernc.org/libc.(*TLS).Alloc(0xc00018daf0?, 0x443d71?)
	/home/runner/go/pkg/mod/modernc.org/libc@v1.29.0/etc.go:248 +0x1a
modernc.org/sqlite/lib.logBadConnection(0x0, 0x79c9ed)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:14577 +0x32
modernc.org/sqlite/lib.Xsqlite3SafetyCheckSickOrOk(0x578b7c?, 0x47b79a?)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:14619 +0x38
modernc.org/sqlite/lib.Xsqlite3SafetyCheckOk(0x7f5f21c003e0?, 0x0?)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:14605 +0x29
modernc.org/sqlite/lib.sqlite3LockAndPrepare(0x791eda?, 0x7f5f21600428, 0x7f5f21e00f20, 0x4137b2?, 0x0?, 0x7f5f20c00000?, 0x7f5f21c003e0, 0x0?)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:86092 +0x52
modernc.org/sqlite/lib.Xsqlite3_prepare_v3(...)
	/home/runner/go/pkg/mod/modernc.org/sqlite@v1.27.0/lib/sqlite_linux_amd64.go:86175
zombiezen.com/go/sqlite.(*Conn).prepare(0xc000074370, {0x791eda, 0x89}, 0x0)
	/home/runner/go/pkg/mod/zombiezen.com/go/sqlite@v1.1.0/sqlite.go:503 +0x275
zombiezen.com/go/sqlite.(*Conn).PrepareTransient(0x791f63?, {0x791eda?, 0x0?})
	/home/runner/go/pkg/mod/zombiezen.com/go/sqlite@v1.1.0/sqlite.go:480 +0x1f
zombiezen.com/go/sqlite/sqlitex.ExecuteTransient(0x784548?, {0x791eda, 0x89}, 0x5ad?)
	/home/runner/go/pkg/mod/zombiezen.com/go/sqlite@v1.1.0/sqlitex/exec.go:198 +0x3e
command-line-arguments.TestCreateAndInitializeDatabase(0xc0001569c0)
	/home/runner/work/phab-migration-tools/phab-migration-tools/bugz/db_test.go:16 +0xe6
testing.tRunner(0xc0001569c0, 0x7bb920)
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/testing/testing.go:1595 +0xff
created by testing.(*T).Run in goroutine 1
	/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.5.linux-amd64/src/testing/testing.go:1648 +0x3ad
FAIL	command-line-arguments	0.006s
FAIL
Error: Process completed with exit code 1.

Is it possible to provide a more user-friendly panic message when a closed DB is being used?
For example put somewhere a check :

if closed {
    panic("The database connection was closed")
}

[ngortheone]

@zombiezen the 3255b67 commit does not really fix the problem, or at least I don't see how can it. It will only give an error on repeated close, but not when a closed connection is used to execute statements. The problem I had was that I was trying to perform a a select on the closed connection.